Cytogenetics has determined the incidence and prognostic significance of chromosomal abnormalities in acute lymphoblastic leukaemia (ALL). The development of fluorescence in situ hybridization (FISH) and array technologies has led to the discovery of novel aberrations. Five 'hot topics' are presented in which cytogenetics and related techniques have been instrumental in understanding the role of genetics in leukaemogenesis: (i) genetic changes are integral to the biology of T-cell ALL; (ii) intrachromosomal amplification of chromosome 21 is a new recurrent abnormality in precursor-B ALL (BCP-ALL); (iii) the immunoglobulin heavy chain gene (IGH@) is significant in BCP-ALL; (iv) alterations in genes involved in B-cell development and cell cycle control contribute to the pathogenesis of BCP-ALL; (v) age-related cytogenetic profiles define ALL in children and adolescents as distinct biological entities. In this molecular era, cytogenetics continues to be integral to our understanding of the genetics of this disease.